The present invention relates generally to the field of analog signal processing circuits, and more particularly to the flied of analog signal processing circuits utilized to detect engine knock conditions in automobile internal combustion engines.
During the operation of automobile internal combustion engines, many times it is necessary to detect the condition of engine knock wherein this condition is characterized by an excessively loud noise level, above the normal background engine vibration noise level for a particular engine speed, being generated by the engine due to excessive engine vibration. Typically, engine knock exists for a relatively long time after each spark ignition, whereas normal engine operation would produce, at most, only short duration, high amplitude noise and would in general produce a low amplitude background noise signal related to engine speed. It is possible to eliminate or minimize engine knock by adjusting the time occurrence of spark ignition for the internal combustion engine. Thus, some prior systems have utilized knock detection circuits to develop control voltages for controlling engine spark timing.
Typically, engine knock sensors, generally corresponding to vibration sensors, have been utilized to detect knock by producing electrical signals in accordance with sensed engine vibrations. During a condition of engine knock, the amplitude of these vibrations will greatly increase in a specific band of "knock frequencies" which are characteristic of the mechanical construction of the internal combustion engine. Prior knock sensor circuits have attempted to detect engine knock through the utilization of analog signal processing circuits which receive the engine knock sensor signal. For various reasons, prior analog engine knock signal processing circuits have proved unsatisfactory due to either their excessive cost and complexity, their failure to adequately distinguish between an engine knock condition and normal engine background noise, or their failure to provide a circuit having an engine knock detection characteristic which can be readily adjusted to provide a desired relationship between the severity of the engine vibrations resulting from engine knock and varying amplitudes of engine vibrations resulting from normal engine operation. Typically, a knock detection characteristic is desired which requires larger amplitude vibrations to indicate a knock condition (excessive engine noise) as normal engine background noise increases, such as would occur when the operating speed of the engine increases.
Some prior knock detection circuits compare the amplitude of the engine vibration signal to a fixed reference level in order to detect engine knock. These circuits do not provide the desired engine knock detection characteristic and they ignore the fact that the amplitude of the engine vibration signal is actually a composite of engine knock signal amplitudes and typical engine vibration noise. Some knock detection circuits have attempted to process the knock sensor signal by effectively compensating for the effect of background noise on the amplitude of the engine vibration sensor signal during engine knock conditions. These circuits have generally been costly and complex and hard to adjust to obtain a specific desired knock detection characteristic for varying amounts of engine background noise. Generally, the prior knock detection circuits have also had problems in adequately distinguishing short duration, high amplitude engine background noise from the longer duration, high amplitude signals characteristic of engine knock.